Total inactivation of ten microorganisms inoculated in a model wastewater was achieved by ozone treatment at laboratory scale. Microorganisms selected included four viruses, three non-sporulated bacteria, two sporulated bacteria and one fungus, employed in vaccine preparation for animals. Viruses resulted in the most readily-inactivated microorganisms and sporulated bacteria the most resistant strains, althoughSalmonella typhimurium, a non-sporulated bacterium, showed an unusually high resistance. Inactivations followed a second-order kinetic law, depending on both ozone and microorganism concentrations. In each case, kinetic parameters which characterize the inactivation process were measured experimentally.